The invention relates to fuel injection pumps for internal combustion engines in which fuel is delivered under pressure to fuel injection valves or nozzles. More particularly, the invention relates to fuel injection pumps in which a driven shaft actuates cams that drive a pump piston and which also include means for adjusting the onset of injection by changing the relative rotary position of the drive shaft of the pump and the cam drive which moves the pump piston. These fuel injection pumps also include control members which control the flow of fuel from a region of relatively low pressure to a pressure chamber connected to the injection valves.
In known fuel injection pumps of the type described above, the onset of injection is changed by shifting the portion of the drive cams which is used for fuel injection. Inasmuch as the shape of the cam surface changes drastically, especially at the beginning and end of the cam, this type of shifting also affects the fuel delivery velocity and the fuel rate and requires compensation for both of these factors to avoid substantial disadvantages. For example, if a shift to earlier injection takes place, the fuel delivery region is shifted in the direction of the onset of the cam surface, i.e., into a region of relatively more shallow cam slope, so that during a predetermined angular rotation of the drive shaft, the fuel delivery piston executes a substantially smaller stroke, thus reducing the quantity of fuel injected. This smaller quantity of fuel is being injected over an extended time period and thus reduces the fuel delivery velocity, especially at the beginning of the cam slope. Generally speaking, an advance of the injection takes place at high engine speeds where an increased injection speed is desired whereas, at low engine speeds, the injection time is preferably increased so as to reduce engine noise. The effect of the known injection advance thus completely diverges from the desired results. Furthermore, if the injection pump is intended to supply a six or eight cylinder engine for example, i.e., if six or eight cams are present on the cam plate, the spatial considerations require the slope of the cams to be relatively steep because eight separate cams may have to be located in a 360.degree. extent of the drive shaft. In such a case, 45.degree. would be available per cam and approximately 20.degree. for each flank. Accordingly, nearly the entire flank must be used for fuel delivery so that a shift of the fuel delivery region can take place only within very narrow limits. Some known injection pumps, for example the simple serial pumps, are provided with shaft-driven injection adjustors which require additional cost and additional constructional space.